0610 · 19.1Energy flow

Energy flow through organisms

0:00A rabbit runs across a field, using energy to move its muscles. It gets that energy from food, such as grass, but where did the grass get its energy? Follow the story backwards, and you reach the Sun. By the end of this video, you will understand how energy enters living systems, passes through organisms and eventually transfers to the environment. The starting point is this statement. The Sun is the principal source of energy input to biological systems.

0:25Principal means main, so we are identifying the main starting point for the energy that supports life. A biological system might be a pond, a woodland or the field containing our rabbit. Sunlight supplies the energy that enters these systems through photosynthesis. Before we follow that energy, remember that energy and matter are different things. Grass contains substances, such as sugars, that store chemical energy. When a rabbit eats grass, it takes in those substances and the chemical energy associated with them.

0:54Energy is not an extra ingredient floating inside the food. It is something that can be transferred and used to bring about changes. Start with the grass growing in sunlight. Chlorophyll absorbs light energy, and photosynthesis uses that energy to make glucose from carbon dioxide and water. The important energy change is from light energy to chemical energy stored in glucose. We do not need to revisit all of photosynthesis here.

1:19We just need to understand that this is how energy enters the living part of the system. The grass can use glucose to make other substances that become part of its body. This means some of the energy captured from sunlight is now stored as chemical energy in plant material. A plant is called a producer because it makes its own organic nutrients, including carbohydrates. It produces these substances, but it does not produce energy from nothing.

1:43Now imagine the rabbit eating the grass. Chemical energy in the plant material is transferred to the rabbit in its food. Some of the substances it absorbs can become part of the rabbit’s body, while others can be broken down to release energy. The rabbit receives chemical energy, not sunlight, from the grass. How does the rabbit use that chemical energy? Respiration involves chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

2:10That energy supports activities such as movement, growth and making new substances. As respiration and other activities take place, energy is also transferred to the surroundings as heat. The grass respires too, because plant cells also need energy for their activities. Photosynthesis captures light energy, while respiration releases energy from nutrient molecules. These are different processes, and plants carry out both. So our diagram needs an arrow showing heat transferred from the grass to the environment, as well as one from the rabbit.

2:39This explains why we describe energy as flowing through living organisms. It enters mainly as sunlight, becomes chemical energy in producers and can pass to other organisms through feeding. Eventually, it is transferred to the environment as heat. Once dispersed into the surroundings, that energy does not return to the grass as light energy for photosynthesis.

Tracing energy in grass and rabbits

3:00Let us work through a complete example using this diagram. The task is to describe the energy changes from sunlight reaching grass to a rabbit eating that grass and running. First, identify how energy enters. It reaches the grass as light energy from the Sun. We begin there because the question asks us to trace the source, not just explain what the rabbit does. Next, identify the change that happens in the grass.

3:24During photosynthesis, light energy is converted into chemical energy stored in glucose and other organic substances made from it. Then follow the feeding arrow. When the rabbit eats grass, chemical energy in the food is transferred to the rabbit. At this stage, the energy remains chemical energy, and eating does not turn it back into light. Finally, consider how the rabbit uses the energy and where it goes. Respiration releases energy from nutrient molecules, allowing the rabbit’s muscles to work.

3:51Energy is transferred to the environment as heat during respiration and activity. Remember to include heat transfer from the grass too, because energy transfer to the surroundings happens in plants as well as animals.

Energy flow mistakes and exam answers

4:03One common mistake is saying that plants get their energy from the soil. Soil supplies water and mineral ions, but the energy input for photosynthesis is light from the Sun. Another mistake is saying that respiration creates energy. Respiration releases energy from nutrient molecules. It does not make new energy from nothing. A third mistake is saying that energy is recycled. Nutrients can be recycled, but energy flows through biological systems and is eventually transferred to the environment as heat.

4:32Saying energy is lost means it is no longer available to be passed on in food, not that it has been destroyed. Biological systems therefore need a continuing energy input, mainly from the Sun. An examination could ask you to state the principal source of energy input to biological systems. For state, give the direct answer. The Sun is the principal source of energy input to biological systems. A describe question about energy flow needs the sequence of transfers.

5:00Follow the diagram in order, naming the energy forms and finishing with the environment. Here is a clear model answer to the describe question. Light energy from the Sun is absorbed by the grass. Photosynthesis converts light energy into chemical energy stored in organic substances. Chemical energy is transferred to the rabbit when it eats the grass. Energy is eventually transferred from the organisms to the environment as heat. Let us bring the whole idea together.

5:26The Sun is the principal energy source, and photosynthesis converts its light energy into chemical energy in producers. Feeding transfers chemical energy between organisms, while respiration releases energy for their activities. Energy eventually transfers to the environment as heat, so it flows through the system rather than being recycled.